Noise rejection in digital signal receivers is always a major consideration in the design of any digital system, particularly those with remote peripherals. Digital transmission signals generally consist of a series of pulses having a uniform pulse width. Noise pulses are generally of short duration and vary in level both above and below the signal pulses.
In order to eliminate the effect of low level noise pulses threshold devices such as Zener diodes or Schmitt triggers that switch on at a voltage slightly lower than the signal pulse amplitude are used in series with the input terminal of a receiver. Such devices generally have only a slight or negligible effect on the pulse width of the signal pulses and eliminate noise pulses that have an amplitude below the switching voltage of the threshold device.
Eliminating short duration high amplitude pulses is a more difficult problem. The basic approach in the prior art was to either compare the input signals with a delayed version of the input signals in an NAND gate or to use an integrating circuit to convert the input pulses to ramp-like signals that increase as a function of time and to input these ramp-like signals into a threshold device.
Both of these methods produce uneven output wave forms and have other drawbacks to be described later.
A third prior art noise elimination method uses the leading edge of the input pulses to trigger two monostable multivibrators or "one shots", one producing an output pulse of a duration slightly longer than the signal pulses and the other providing an output pulse slightly shorter in duration than the signal pulse. The output of these one shots are compared in an exclusive OR gate to produce a gating pulse for an AND gate into which the original input pulse is connected. If the input pulse is of sufficient duration to persist during the gating pulse, then it is a signal pulse and the AND gate produces a triggering signal for a third one-shot that provides an output having a pulse width calculated to be the same as that of the signal pulses. Such a noise eliminator must be designed for a single pulse width and, if provided with a noise pulse immediately preceding a signal pulse, can result in an output pulse with a leading edge corresponding to the leading edge of the noise pulse. This shift of the leading edge could, of course, be remedied if the one-shots were retriggerable, but the problem of designing each set of one-shots for a particular signal pulse width would remain.